Sanja Vidacek , Helga Medic, Nives Marusic, Božena Surić and Tomislav Petrak

1. IntroductionIn recent years, bioelectrical properties of frozen-thawed fish have attracted much attention as possible indicators of freezing history. In our previous studies, the resistance and reactance of frozen-thawed sea bass and rainbow trout in a frequency range 100 Hz-1 MHz were measured. The results revealed that the reactance higher than 500 kHz could differentiate the fish with different freezing history. In this work we assessed whether frozen-thawed, commercially important wild fish Atlantic chub mackerel had similar bioelectrical properties as farmed species. Influence of different freezing regimes on bioelectrical properties of Atlantic chub mackerel (Scomber colias) Results Materials and Methods EXPERIMENTAL PLAN Thirty (30) samples of Atlantic chub mackerel a CLEANING, FILLETING CHILLING (+4 C,12 hours) CUTTING THE EPAXIAL MUSCLE (10 x 2 x 1 cm) DIVIDING THE SAMPES 2. group of samples UNFROZEN (CHILLED) • group of samples • UNFROZEN (CHILLED) b Bioelectrical measurements Resistance (R) and Reactance (X) measured by HPLCR Meter at 19 frequencies from 0.1-1000 kHz; constant current method Physical and chemical measurements Water holding capacity (WHC), water content, pH - SLOW FREEZING (AIR,-18C) FAST FREEZING (LIQUID N2) Figure 1Resistance (R) and reactance (X) spectra of slow frozen (a) and fast frozen (b) groups of Atlantic chub mackerel FROZEN STORAGE (-18 C, 14 days) Table 1Differences between slow and fast frozen samples after one and two freezing cycles (Mann Whitney U test) THAWING BY AIR (+4 C,12–16 hours) Sanja Vidacek, Helga Medic, Nives Marusic, Božena Surić and Tomislav Petrak Faculty of Food Technology and Biotechnology, University of Zagreb, Pierottijeva 6, Zagreb, Croatia corresponding author:svidacek@pbf.hr SLOW FROZEN-THAWED SAMPLES FAST FROZEN-THAWED SAMPLES Repeated bioelectrical, physical and chemical measurements Legend Sf-slow frozen SfII-double Sf Ff – fast frozen FfII--double Ff SLOW FREEZING (AIR,-18C) FAST FREEZING (LIQUID N2) FROZEN STORAGE (-18 C, 14 days) THAWING BY AIR (+4 C,12–16 hours) DOUBLE FAST FROZEN-THAWED SAMPLES DOUBLE SLOW FROZEN-THAWED SAMPLES Table 2.ANOVA among unfrozen, frozen and double frozen samples Repeated bioelectrical, physical and chemical measurements STATISTICAL ANALYSIS (SPSS, 12.0) Kolmogorov Smirnov test Confirmation of the normality of data Testing the difference between -fast and slow frozen samples Mann Whitney U test -double fast and double slow frozen samples -unfrozen, frozen and double frozen samples ANOVA ConclusionsThe relationship between frequency and bioelectrical parameters follows the same pattern for all the fish species studied previously. Mann-Whitney U test shows that that the reactance is the only parameter that can differentiate fast frozen from slow frozen fish and double fast frozen from double slow frozen fish. The results of testing the differences among unfrozen, frozen and double frozen samples (ANOVA) show that the resistance measured at 500 and 1000 kHz, reactance measured between 0.1 and 15 kHz, moisture content and WHC are significantly different for different tissue conditions. Therefore, the reactance is the only variable that can distinguish fast frozen from slow frozen fish samples (at higher frequencies) and that may differentiate between double and single freezing cycle (at low frequencies) which supports our previous results on farmed fish.